Previous studies led to the working hypothesis concerning the mechanism underlying the presentation and recognition of the cell-surface Thy-1 alloantigens. According to this hypothesis, two distinct forms of the Thy-1 antigen-cell-bound or cell-free are presented in the context of donor's class I or responder's class II combinatorial H-2 molecules, respectively. In an attempt to confirm and elaborate the working hypothesis, the experiments will be carried to achieve the following aims: (1) identifying either IA genotypes that influence the anti-Thy-1 response to the cell-free antigen, (2) characterizing the T cell subset(s) involved in anti-Thy-1 response elicited by different forms of Thy-1 antigen, (3) defining other mechanisms that may affect anti-Thy-1 response, (4) identifying a specific hybrid IA determinant involved in anti-Thy-1 response. Anti-Thy-1 response will be induced by injection of Thy-1 disparate thymocytes from various donors into appropriate responders (inbred mice, F1 hybrids, H-2 recombinants. The magnitude of the anti-Thy-1 response in various experimental combinations will be determined by enumerating anti-Thy-1 plaque-forming cells and measuring titer of serum antibodies. The cells from animals primed in vivo or cells primed in vitro will be tested by T cell proliferation assay for their specificity for a given form of Thy-1 antigen. These cells will be characterized for their Lyt phenotypes either prior or after cloning in vitro. The identification of the hybrid IA determinant will be attempted by secondary MLR, hyperimmunization in vivo and/or production of specific hybridoma. If specific antibodies (standard or monoclonal) are obtained, their effect on anti-Thy-1 response in vivo and/or in vitro will be tested. The long-term objective of the proposed studies is to define as precisely as possible the mechanism underlying the primary anti-Thy-1 response as the model for the responses to other cell-surface alloantigens.